3GPP (3rd Generation Partnership Project) specifications define the function for reducing location registration procedures when a mobile station makes reselection between different RANs (Radio Access Networks) provided by different RATs (Radio Access Technologies). This function is referred to as ISR (Idle mode Signalling Reduction).
RAN covers a geographical area divided into a plurality of cells or sectors. Each cell or sector is provided by a base station. Specific examples of RAN include UTRAN (Universal Terrestrial Radio Access Network), GERAN (GSM EDGE Radio Access Network) and E-UTRAN (Evolved Universal Terrestrial Radio Access Network). A base station is referred to as NodeB, BTS (Base Transceiver Station) or eNodeB, for example. Further, a mobile station is referred to as UE (User Equipment) or MS (Mobile Station), for example.
The term “RAT” is used as synonymous with RAN in some cases; however, it is a concept that includes not only RAN but also a mobility management node or entity placed in a core network in general. Specific examples of RAT include UTRA (Universal Terrestrial Radio Access) and E-UTRA (Evolved Universal Terrestrial Radio Access). E-UTRA is also called LTE (Long Term Evolution) in some cases. A mobility management node or entity in UTRA is SGSN (Serving GPRS Support Node). A mobility management node or entity in E-UTRA is MME (Mobility Management Entity).
Typically, the ISR can reduce signalling of location registration procedures when a mobile station in the idle state makes cell reselection between different RATs. The idle state (or idle mode) generally means the following state. Specifically, in the idle state, a mobile station is attached to a core network, and both of the mobile station and the mobility management node (or entity) have MM contexts (or MM parameters). However, a NAS (Non Access Stratum) message is not sent and received between the mobile station and the mobility management node, and dedicated radio resources in the RAN are not allocated for the mobile station. The core network keeps track of the location of the mobile station in units of location registration area. The location registration area is called RA (Routing Area) in UTRA and called TA (Tracking Area) in E-UTRA. Note that, in E-UTRA, the mobile station can belong to a plurality of TAs. In this case, the mobility management node (i.e., MME) keeps track of the location of the mobile station in units of TA list. The TA list can contain a plurality of TAs. Specific examples of idle state include ECM (EPS Connection Management)-IDLE state in E-UTRAN, PMM (Packet Mobility Management)-IDLE state in UTRAN and GPRS-STANDBY state in GERAN. Further, specific examples of idle state may include at least one of RRC_IDLE state in E-UTRAN, CELL_PCH state and URA_PCH state in UTRAN, and Packet Idle state in GERAN.
When the mobile station frequently makes reselection between different RANs, e.g., between E-UTRAN and GERAN/UTRAN, a location registration procedure is executed each time. The location registration procedure is referred to as a location update procedure in some cases. The location registration procedure in E-UTRAN is TAU (Tracking Area Update). The location registration procedure in GERAN/UTRAN is RAU (Routing Area Update). The location registration procedure involves signalling between the mobile station and the mobility management node located in the core network and signalling in the core network. Signalling in the core network includes, for example, signalling between the mobility management node and a subscriber server (e.g., HSS (Home Subscriber Server), HLR (Home Location Register)). Accordingly, frequent location registration procedures cause an increase in network load.
The ISR aims at reducing the location registration procedures caused by reselection between different RANs (Radio Access Networks). Currently, 3GPP defines ISR for reselection by the mobile station between E-UTRAN and GERAN/UTRAN. When the ISR is activated, the mobile station is registered in both of the two mobility management nodes (or entities) corresponding to two RANs (or RATs). To be specific, the mobile station is registered in both of the MME and the SGSN. Further, the mobile station has both of the MM (Mobility Management) contexts received from the SGSN and the MM contexts received from the MME. The MM contexts from the SGSN contain P-TMSI (Packet-Temporary Mobile Subscriber Identity) and RAI (Routing Area Identity), for example. The P-TMSI is a temporary identifier assigned to the mobile station by the SGSN. The MM contexts from the MME contain GUTI (Globally Unique Temporary Identity) and TAI (Tracking Area Identity), for example. The GUTI is a temporary identifier assigned to the mobile station by the MME. When the ISR is activated, the mobile station can make reselection between two RANs (i.e. GERAN/UTRAN and E-UTRAN) without performing a location registration procedure (i.e., RAU, TAU) unless it moves out of the two location registration areas (i.e. RA and TA(s)) registered in the network.
The ISR is activated in the following procedure, for example. First, the mobile station performs initial attach through either one of two RANs, i.e., through GERAN/UTRAN or E-UTRAN in this example. Here it is considered that the mobile station performs initial attach through E-UTRAN. In the initial attach, the mobile station performs a location registration procedure with the MME, and the mobile station is registered in the MME. The mobile station enters ECM-IDLE state.
Next, when the mobile station first reselects GERAN/UTRAN, the mobile station initiates a location registration procedure (i.e., RAU). Specifically, the mobile station sends a RAU request to the SGSN. The RAU request contains P-TMSI mapped from GUTI. The SGSN sends a context request to the MME specified from the received GUTI. The SGSN thereby sends the MM contexts (e.g., P-TMSI, RA) of the mobile station managed by itself to the MME, and receives the MM contexts (e.g., GUTI, TA(s)) from the MME. Then, the SGSN sends a RAU accept. The RAU accept carries the MM contexts (e.g., P-TMSI, RA) and indicates “ISR activated”.
Upon receiving the RAU accept indicating “ISR activated”, the mobile station activates the ISR in the own station. To be specific, the mobile station sets its TIN (Temporary Identity used in Next update) to “RAT-related TMSI”. The TIN is one of the MM contexts held by the mobile station. The TIN defines the identifier of the mobile station to be presented by the mobile station in the next location registration procedure, i.e., RAU request, TAU request or attach request. As a result of setting the TIN to “RAT-related TMSI”, the ISR operation in the mobile station is started.
For overview of the ISR, refer to §4.3.5.6 “Idle mode signalling reduction function” and Annex J “High Level ISR description” in Non Patent Literature 1 (3GPP TS 23.401). The ISR is also described in Non Patent Literature 2 (3GPP TS 24.301) and Non Patent Literature 3 (3GPP TS 24.008).
Further, 3GPP specifications define a plurality of methods such as CS Fallback and IMS Voice (Voice over IP) to provide a voice service to the mobile station in E-UTRA. In relation to this, the network (e.g., MME) can indicate which RAN (or RAT) is to be selected preferentially when the mobile station that supports the voice service selects a cell. For example, “UE's usage setting” and “Voice domain preference for E-UTRAN” that are sent from the mobile station to the network may be used as an index to determine the RAN (or RAT) to be selected preferentially. The “UE's usage setting” and “Voice domain preference for E-UTRAN” are stored in the mobile station and sent to the mobility management node (i.e., MME, SGSN) by TAU request, RAU request and attach request.
The “UE's usage setting” indicates whether the mobile station behaves in “a voice centric” or “a data centric”. The “Voice domain preference for E-UTRAN” indicates whether the mobile station supports CS (Circuit Switched) voice or PS (Packet Switched) voice and indicates which one of CS voice and PS voice is more preferred. To be specific, the “Voice domain preference for E-UTRAN” indicates the mobile station is configured as “CS Voice only”, “CS Voice preferred and IMS PS Voice as secondary”, “IMS PS Voice preferred and CS Voice as secondary” or “IMS PS Voice only”.
For overview of the “UE's usage setting” and “Voice domain preference for E-UTRAN”, refer to §4.3.5.9 “Voice domain preference and UE's usage setting” and §4.3.6 “Radio Resource Management functions” in Non Patent Literature 1 (3GPP TS 23.401), for example. The “UE's usage setting” and “Voice domain preference for E-UTRAN” are also described by Non Patent Literature 2 (3GPP TS 24.301) in §3.1 “Definitions”, §4.3 “UE mode of operation”, §5.5.3.2.2 “Normal and periodic tracking area updating procedure initiation”, §5.5.3.3.2 “Combined tracking area updating procedure initiation” and the like. Further, they are described also by Non Patent Literature 3 (3GPP TS 24.008) in §4.7.5.1 “Normal and periodic routing area updating procedure”, §4.7.5.2.1 “Combined routing area updating procedure initiation” and the like.
In this specification, the term “voice service configuration information” is defined as information that is related to the priority of a voice service stored in the mobile station and that is sent from the mobile station to the network and used in the network as an index for selecting RAN (or RAT) which provides a voice service to the mobile station, such as the above-described “UE's usage setting” and “Voice domain preference for E-UTRAN”.